1. Field of the Invention
The present invention relates to image forming apparatuses for use in electrophotographic copying machines and color printers, in which the toner adhered to transfer members is removed.
2. Description of the Related Art
Some image forming apparatuses of copying machines and printers have a transfer member that comes into contact with an opposed image bearing member to transfer toner images formed on the image bearing member. The apparatuses pass a sheet of paper between the image bearing member and the transfer member, and form an electric field between the image bearing member and the transfer member to transfer the toner image on the image bearing member onto the sheet of paper.
However, in this transfer system, without a sheet of paper between the image bearing member and the transfer member, the transfer member comes into direct contact with the image bearing member to cause the toner to adhere to the transfer member, smudging the surface of the transfer member. Specifically, toner adhered by paper jamming, fog toner on the image bearing member, or the toner of test images such as resist marks may adhere to the transfer member to smudge the surface of the transfer member. The toner adhered to the transfer member will smudge the back of the sheet of paper during the succeeding transfer or, for both-side printing, will smudge the surface of the images.
Therefore, some known apparatuses apply a bias voltage to the image bearing member or the transfer member during handling of paper jamming to form an electric field between the image bearing member and the transfer member, thereby recovering the toner adhered to the transfer member to the image bearing member. With those apparatuses, the resistances of the image bearing member and the transfer member vary with the change of environment. Accordingly, to apply a more suitable bias voltage to the image bearing member or the transfer member, the known apparatuses have controlled the bias voltage to be applied to the image bearing member or the transfer member according to the measurements by a temperature and humidity sensor.
However, the resistances of the image bearing member and the transfer member are influenced not only by the changes of temperature and humidity but also by the frequency or condition of use of the apparatuses. For example, the surface characteristic of the image bearing member or the transfer member changes by continuous transfer, double-sided transfer, or the like to cause the changes of the resistances. Accordingly, there is the possibility that a bias voltage necessary for recovering the toner adhered on the transfer member to the image bearing member cannot be obtained even when the bias voltage is controlled according to the measurements by the temperature and humidity sensor. Accordingly, the toner adhered on the surface of the transfer member may not be sufficiently recovered, causing smudges on the back of a sheet of paper or images.
Accordingly, it is desired for image forming apparatuses that transfer images by bringing an image bearing member and a transfer member into contact with each other to prevent smudges on the back of recording media or images without fail by smoothly recovering the toner adhered to the transfer member.